1. Field of the Invention
The present invention relates to a corona charger for use in electrophotographic image forming apparatuses. More particularly, the present invention relates to a scorotron corona charger including a grid electrode. In addition, the present invention also relates to a process cartridge and an image forming apparatus including the scorotron corona charger.
2. Discussion of the Related Art
In a typical electrophotographic image forming apparatus, first, a surface of a photoreceptor is evenly charged, and the charged surface is then exposed to a light beam modulated by image information to form an electrostatic latent image thereon. A toner is supplied to the electrostatic latent image to form a toner image on the surface of the photoreceptor. The toner image is transferred onto a recording medium directly or via an intermediate transfer member, and then fixed thereon upon application of heat and pressure. Residual toner particles remaining on the surface of the photoreceptor are removed by a cleaning blade.
Corona chargers are typically used for charging photoreceptors.
Corona discharge is a continuous discharge phenomenon that occurs upon local dielectric breakdown of air in an uneven electric field. A typical-corona charger has a configuration in which a corona wire with a micro-diameter is stretched taut in a shield case made of aluminum, a part of which is eliminated. Corona ions are discharged from the part of the shield case which is eliminated. As the voltage applied to the corona wire increases, a strong electric field is locally formed at the periphery of the corona wire, causing local dielectric breakdown of air and thus continuous discharge of electricity.
The type of corona discharge largely depends on the polarity of the voltage applied to the corona wire. A positive corona discharge causes an even electric discharge on the surface of the corona wire, whereas a negative corona discharge causes a local streamer discharge. Accordingly, the positive corona discharge has an advantage over the negative corona discharge in evenness of electric discharge. In addition, the negative corona discharge produces several tens of times the amount of ozone produced by the positive corona discharge, thereby increasing environmental load.
FIG. 1A is a schematic view illustrating an embodiment of a corotron corona charger. A charging wire made of tungsten with a diameter of 50 to 100 μm is shielded with a metal case forming a gap of about 1 cm therebetween. A high voltage of 5 to 10 kV is applied to the wire, while an opening is disposed facing a charging target. Thus, positive or negative ions are moved to a surface of the charging target, resulting in charging of the charging target.
FIG. 2A is a graph showing a relation between the charging time and the surface potential of a charging target with respect to the corotron corona charger. It is apparent from FIG. 2A that the corotron corona charger continuously charges the charging target, in other words, continuously discharges electricity. Therefore, the corotron corona charger is not always suitable for charging a charging target to a predetermined potential, whereas it is suitable for constantly charging a charging target.
FIG. 1B is a schematic view illustrating an embodiment of a scorotron corona charger. The scorotron corona charger was developed for the purpose of reducing unevenness in the resultant potential of a charging target. As illustrated in FIG. 1B, the scorotron corona charger has a configuration in which a plurality of wires or a mesh is provided as a grid electrode in an opening of the metal shield case. The opening is disposed facing a charging target, and a bias voltage is applied to the grid electrode.
FIG. 2B is a graph showing a relation between the charging time and the surface potential of a charging target with respect to the scorotron corona charger. It is apparent from FIG. 2B that the surface potential of the charging target is saturated at a predetermined charging time. This is because a voltage applied to the grid electrode controls the surface potential of the charging target. The saturation value depends on the voltage applied to the grid electrode.
Although having a more complicated configuration and providing a lower charging efficiency than the corotron corona charger, the scorotron corona charger is widely used because of having an advantage in evenness of charging. The grid electrode may be hereinafter described as “charging grid” also.
It is known that both corotron and scorotron corona chargers typically produce discharge products such as O3, NOx, nitrate ion, and ammonium ion, because substances in the air such as oxygen atoms and nitrogen atoms are reacted upon a high-voltage discharge of from 5 to 10 kV. These discharge products may adhere to or permeate in a photoreceptor (i.e., charging target), and therefore abnormal images with white spots, black bands, blurring, etc., may be disadvantageously produced.
In attempting to solve such problems, Unexamined Japanese Patent Application Publication No. (hereinafter “JP-A”) 2005-227470 discloses a corona charger, the charging grid of which is made of stainless steel and coated with a conductive coating composition including an organic binder resin and fine particles of graphite, nickel, and an aluminum compound. It is disclosed therein that such a configuration prevents corrosion of the charging grid because the conductive coating layer adsorbs discharge products. Accordingly, a charging target is prevented from being contaminated with discharge products. However, since the fine particles in the conductive coating layer adsorb discharge products, the capacity for adsorbing discharge products depends on the number of adsorbing sites in the fine particles, and there is a possibility that the adsorbing sites become buried with long-term use.
Unexamined Japanese Utility Model Application Publication No. 62-089660 discloses a corona charger in which finely partitioned communicating holes are arranged within an opening, and an ozone-adsorbing layer containing an ozone-adsorbing material is further formed on the inner surface of the communication holes. A zeolite and an activated carbon are used as the ozone-adsorbing material. It is disclosed therein that such a configuration prevents diffusion of ozone. However, it is difficult to prevent ozone from diffusing toward a charging target side, possibly contaminating a charging target with ozone.
JP-A 2003-43894 discloses an image forming apparatus including a corona charger and a means for removing (adsorbing) discharge products adhered to a charging target, and at least one of a means for preventing adhesion of discharge products to the charging target, a means for preventing lowering of the resistance of the discharge products adhered to the charging target, and a means for reducing the amount of discharge products produced at the periphery of the charging target. Accordingly, multiple members are needed, which is disadvantageous. An embodiment is also disclosed therein in which an adsorbent such as a zeolite is provided between the charging target and the corona charger. However, such an embodiment cannot reliably charge the charging target.